for (i = 0; i < 5; i ++) {
gettimeofday(&sleep_until, NULL);
sleep_until.tv_sec += 5;
gettimeofday(&last, NULL);
do {
count++;
gettimeofday(&now, NULL);
if ((now.tv_sec == last.tv_sec) && (now.tv_usec ==
last.tv_usec))
fail ++;
else if (timercmp(&last, &now, >))
back ++;
last.tv_sec = now.tv_sec;
last.tv_usec = now.tv_usec;
} while (timercmp(&now, &sleep_until, <));
printf("Ran gtod() %lu times in 5 seconds.\n", count);
printf("%f gtod()/usec\n", (float)count / 5000 / 1000);
printf("Failed to increment %lu times\n", fail);
printf("Back in time %lu times\n", back);
printf("-------------\n");
count = 0;
fail = 0;
back = 0;
}
exit(0);
}
---- code ----
gcc -O3 -o gtod gtod.c
Very interesting results:
Ran gtod() 68160730 times in 5 seconds.
13.632146 gtod()/usec
Failed to increment 63197136 times
Back in time 0 times
[...etc...]
Interesting how gettimeofday() fails to increment around 90% of the
time. I guess technically incrementing 1/10 times while requiring
~13 times to measure a usec is good enough, but it's not giving me a
warm & fuzzy feeling.
I don't exactly know what you expect here.
% expr 68160730 - 63197136
4963594
Of the 5 million unique microsecond values to be experienced in your
five second test, you saw all but about 36,000 (0.07%) of them.
Or are you dissatisfied that an iteration of your loop takes nearly
80ns to run?
May just be one of those things where a RTOS is necessary to get
any improvement.
In what direction are you looking for "improvement"? It's a common
mistake to think that RTOS' have better resolution; typically what you
get from an RTOS is determinism, often achieved at the cost of peak
performance.
Anyone know if OS X/Darwin has the equivalent of Linux's
RT scheduler extensions?
Darwin has a fairly modest but effective set of RT-like scheduling
capabilities. What is it exactly that you are trying to achieve?
It's very hard to get any sort of "RT" in a system where you have any
sort of competition, and a desktop Darwin system is very competitive.
Anyways, I'm going to try to dig around some more and try to find the
HPET timer API. Theoretically, that should be more accurate then what
I'm seeing with gtod().
What do you mean by "accurate"? Your test above has demonstrated that
gettimeofday is returning unique microsecond values; it has no greater
resolution. If you were truly pedantic you could add roughly half
another significant figure (on your test system) by counting the
number of iterations since you last saw gettimeofday() wrap, but even
then the question around that sort of timing resolution is - what do
you expect it to tell you? Doing any sort of real work takes orders
of magnitude longer than that...
If you think that the HPET will help you, I'm sorry; it's too slow,
it's too far away (the connection is highly latent, you can be held
off talking to it for milliseconds in pathalogical cases), and the
Darwin kernel owns it entirely.
If I understood what you were actually trying to do, I might have some
better ideas...
= Mike
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